Chronology of a probable neotectonic Pleistocene rock avalanche

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Quaternary International 148 (2006) 138–148
Chronology of a probable neotectonic Pleistocene rock avalanche,
Cordon del Plata (Central Andes), Mendoza, Argentina
Stella M. Moreiras
Instituto Argentino de Nivologı´a, Glaciologı´a y Ciencias Ambientales (IANIGLA) – CRICYT, CONICET, Av. Dr. Ruiz Leal s/n,
Parque, (5500) Mendoza, Argentina Casilla Correo 330
Available online 19 January 2006
Abstract
Placetas Amarillas-1 rock avalanche located at 321430 S–691250 W was geomorphologically, stratigraphically and chronologically
studied. This extraordinary event dammed a secondary gully causing the formation of a barrier-lake, indicated by relict lacustrine and
diatomite deposits. Stratigraphically, its deposit is overlain by alluvial fans where three tephra layers are intercalated. Dating using
Ar–Ar method of the middle ash level at 350780 ka determined a Middle Pleistocene or older age for this event. Furthermore, the
younger age of Placetas Amarillas-2 is established by relative dating techniques such as soil development and rock varnish development.
The ages established in this research indicate that Placetas Amarillas-1 may be temporally correlated with the Tigre Dormido rock
avalanche, suggesting that the occurrence of these rock avalanches may be related to regional neotectonic activity. Nevertheless, climatic
conditions are not underestimated as the Tigre Dormido event occurred before formation of an outwash terrace related to a glaciation of
Early Middle Pleistocene age.
r 2005 Elsevier Ltd and INQUA. All rights reserved.
1. Introduction
Rock avalanches, known also as sturzstroms, are large,
extremely rapid and often open-slope flows (Cruden and
Varnes, 1996). Relatively large volumes and extremely high
velocities associated with this kind of flow activity are often
linked to a catastrophic impact. Historical examples
included the Goldau slide of 1806 (volume 30–40 Mm3,
estimated velocity 70 m/s, 457 deaths); the Elm slide of
1881 (volume 11 Mm3, estimated velocity 70 m/s, 115
deaths); and the Frank slide of 1903 (volume 30 Mm3,
estimated velocity 28 m/s, 70 deaths) (Heim, 1919, 1932;
Abele, 1974; Cruden and Krahn, 1978; Hsü, 1978; Cruden
and Hungr, 1986). The research community has a
particular interest in the survey of these huge landslides,
concerning probable causes, rupture mechanisms, velocities, magnitudes, volumes and probable relations with
certain climatic conditions (Panizza, 1973; Abele, 1974;
Hsü, 1975; Plafker and Ericksen, 1978; Voight and
Corresponding author. Tel.: +54 0261 4287029;
fax: +54 0261 4285940.
E-mail address: [email protected].
Pariseau, 1978; Adams, 1981; Záruba and Mencl, 1982;
Keefer, 1984; Gonzalez Diez et al., 1996; Poschinger and
Haas, 1997; Trauth and Strecker, 1999; Corsini et al., 2000;
Hermanns et al., 2000, 2001; Trauth et al., 2000; Borgatti
et al., 2001; Corsini et al., 2001).
As the study area is a seismic region, several landslides
have been triggered by earthquakes with magnitudes higher
than 3.5, and others are frequently related to intense
rainstorms (Moreiras, 2003a, 2004a, 2005a,b). Even though
debris flows and rockfalls are more common events, huge
rock avalanches have affected the region (Salomón, 1979;
Espizúa and Bengochea, 1991; Fauqué et al., 2000, 2001;
Moreiras, 2003b, 2004b). These extraordinary events have
usually dammed valleys and represent a potential hazard
for this developing mountain area, where intense tourist
activity exists. As well, the International road to Chile,
an important traffic route, and the older International
Transandino Railway pass through the area.
Although, these huge landslides have been geomorphologically studied, numerical dating of their deposits and
assessment of their probable causes are still lacking.
Establishing the chronology of these exceptional landslides
is essential to understand their causes and relationship with
1040-6182/$ - see front matter r 2005 Elsevier Ltd and INQUA. All rights reserved.
doi:10.1016/j.quaint.2005.11.009
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S.M. Moreiras / Quaternary International 148 (2006) 138–148
past climatic conditions. Brundsen (1979) remarked that
different hypothesis on the nature of rock avalanche
movement mechanism have been proposed, unfortunately
without complete satisfactory explanation; and lacking on
precise triggering causes do not allow or enable realistic
prediction of these threatening events. Present study
enhances the knowledge of two extraordinary events
identified in the study area. Chronology established for
one of them, Placetas Amarillas-1 rock avalanche, allow
temporal correlation with other rock avalanche studied
previously, suggesting they may be sourced by the same
triggering factor and at the same conditions. Probable
triggering factor is analyzed regarding potential hazard
evaluation.
2. Location of the study area
The study area is located in north-western Mendoza
province at 321430 S latitude and 691250 W longitude. It
comprises a 33 km2 area along Placetas Amarillas. It is
139
situated near La Quinta place, directly south of the
Uspallata railway station (Fig. 1), access to the area is
via International highway N1 7 to Chile, and then
following unconsolidated road along La Quinta gully from
the old Uspallata railway station. Tourist facilities and a
small artificial dam have been built in La Quinta.
3. Geological/geographical setting
The study area comprises the Cordillera Frontal
geological province, characterized with strong relief and
very steep slopes. The highest elevations are: Colorado
(4790 m.a.s.l.), Division (4603 m.a.s.l.), Burro (4293
m.a.s.l.), and Minero (3813 m.a.s.l.) peaks.
A Permo-Triassic volcanic complex (Choiyoi Group)
crops out in the region, composed of pyroclastic material,
lavas, subvolcanic and intrusive rocks. The Tambillos and
Horcajo Formations can be distinguished. The former is
mainly constituted by volcanites, rhyolitic lavas and
lacustrine sediments (Cortés, 1985). The latter corresponds
Fig. 1. Location of study area, where six huge landslides are identified: (a) Tigre Dormido (TD); (b) Placetas Amarillas 1 (PA-1); (c) Placetas Amarillas 2
(PA-2), c-Piedras Blancas 1 (PB-1); (d) Piedras Blancas 2 (PB-2); (e) Piedras Blancas 3 (PB-3), and (f) Burro (BU) (after Fauqué et al., 2000, 2001).
Pleistocene drifts identified in the area are the Uspallata terminal moraine (UM), Uspallata outwash (UO), and Punta de Vacas outwash (PO) (after
Espizúa, 1993; Moreiras, 2003, 2004b). Placetas Amarillas fault (PA) and Piedras Blancas fault (PB) are represented.
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S.M. Moreiras / Quaternary International 148 (2006) 138–148
to rhyolitic—dacitic pyroclastic rocks and lavas (Mirré,
1966). The Choiyoi Group subvolcanic rocks are represented by rhyolitic porphyries and andesitic dikes. A larger
porphyry body outcrops in the Colorado and Minero
peaks, dated to 278710 Ma (Caminos et al., 1979) by K/Ar
(Early Permian). Intrusive rocks are constituted by small
granitic and gabbroic bodies dated at 244710 Ma by K/Ar
(Caminos et al., 1979).
Quaternary sediments are formed by alluvial and
colluvial deposits. Moreover, in this sector, two outwash
terraces can be identified along the Mendoza river; one is
related to the Punta de Vacas Glaciation, correlated with
isotopic oxygen stage 6 (Espizúa and Bigazzi, 1998) and the
other outwash terrace is related to the Uspallata Glaciation, considered as at least Early Middle Pleistocene
(Espizúa, 1993). A terminal moraine belonging to the
Uspallata Glaciation was identified in the Uspallata valley
(Espizúa, 1993) (Fig. 1). Several landslide deposits have
been also identified along the Mendoza river valley
(Moreiras, 2003a, 2004a,b, 2005a).
4. Tectonic framework
Convergence of Nazca and Sudamericana plates, located
300 km westward, generated a compressive regimen that
has been maintained until present. At the latitude of the
survey area, a flat segment of the Nazca Plate approximately 100 km deep has been linked to an intense intraplate
seismic activity and notable neotectonic activity (Bastı́as
et al., 1993; Ramos, 1993). Although historical material
prior to the conquest period and the following three
centuries is scarce, earthquakes with magnitudes greater
than seven have been reported in the region during the last
two centuries (INPRES, 1993) (Fig. 2).
This sector of the Central Argentine Andes is characterized by active tectonic uplift (Ramos, 1993). Structurally,
the study area is characterized by thrust tectonics along the
eastern margin of the Cordillera Frontal, evidenced by
reverse faults of the Carrera fault system called the Espolón
de la Carrera by Polanski (1958). According to Kozlowski
et al. (1993), this fault system is responsible for the uplift of
the Cordillera Frontal during Pliocene–Lower Pleistocene.
The Placetas Amarillas and Piedras Blancas faults, related
to this system, affect the region with eastern vergence. The
former has a northern–southern trend dipping 57–731W
and putting in contact intrusive and volcanite rocks with
Tertiary sediments. The Piedras Blancas fault has a
north–south trend at the southern end, and trends northwest–southeast near Tabolango pampa dipping 54–571W
(Cortés, 1993).
5. Methodology
In the Placetas Amarillas area, at least two landslide
deposits were initially identified by photointerpretation of
air photos (1: 50,000 scale) and satellite images (Fauqué
et al., 2000, 2001). They were classified as rock avalanches.
However, in this study this classification for one of them is
re-assessed based on the Multilingual Landslide Glossary
(WP/WLI, 1993), Hutchinson (1988) and Angeli et al.
(1996). The events were studied in detail from a stratigraphic point of view. Identification and Ar–Ar dating of
an ash level intercalated in alluvial fans that overlies the
deposit of PA-1 rock avalanche allowed determination of
the maximum age for this event.
Relative-age criteria such as soil development and rock
varnish development were used to establish the tentative
chronology of landslide events in the Placetas Amarillas
(Blackwelder, 1931; Sharp and Birman, 1963; Sharp, 1969,
1972; Birkeland, 1973; Burke and Birkeland, 1979; Colman
and Pierce, 1986; Ritter, 1987; Wells et al., 1987a,b;
Knuepfer, 1988). Soil profiles were analyzed in natural
cuts and were classified according to procedures and
terminology suggested by Birkeland (1984). Horizon size
distribution was determined using 0.34 mm and 0.20 mm
sieves, and retained fractions were weighed using a
electronic balance (Metter PN 1210) with 10 mg accuracy.
The fraction smaller than 0.2 mm was analyzed by the
dispersion method. Carbonate content was determined by
a volumetric method, and a Orion pehachimeter with 5, 7
and 8 patrons was used for pH determination. Effervescence grade was established with hydrochloric acid (1 N).
Rock varnish development was observed on boulders
larger than 10 cm randomly selected within an area of
10 10 m. Boulders were classified as: unvarnished: varnished surface less than 10%; slightly varnished: varnished
surface more than 10% and less than 50%; moderately
varnished: varnished surface between 50–90%; and varnished: more than 90% of boulder surface varnished.
6. Previous work
In the study area, huge volumes of debris material were
mobilized in proximal areas. Fauqué et al. (2000) identified
six rock avalanches: (a) Tigre Dormido (TD), (b) Placetas
Amarillas 1 (PA-1), (c) Placetas Amarillas 2 (PA-2),
(d) Piedras Blancas 1 (PB-1), (e) Piedras Blancas 2 (PB-2),
and (f) Piedras Blancas 3 (PB-3) (Fig. 1). Initially, these
deposits were mapped as tectonic breccias during research
on Geological Sheet 3369-15 Potrerillos (Cortés, 1993;
Folguera et al., 2000). Fauqué et al. (2000, 2001) proposed
that the triggering factor of these rock avalanches was a
seismic movement associated with neotectonic activity of
Placetas Amarillas and Piedras Blancas faults. As well,
these authors suggested that rock avalanches occurred
during Upper Middle Pleistocene–Holocene. They observed an ash level underlying rock avalanche deposits
and tentatively correlated it with an ash level dated
360736 ka by Espizúa (1993).
Pleistocene glaciations occurred along Rio Mendoza
valley, have been studied by Espizúa (1993) from Aconcagua peak to Uspallata valley. This author recognized two
outwash terraces related to the Punta de Vacas and
Uspallata Pleistocene glaciations in Uspallata valley. Later,
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141
Fig. 2. (a) Seismic zone linked to flat subduction segment of Nazca plate; and (b) Earthquakes from XX century and some from XVI century (after
Moreiras, 2004a,b).
Moreiras (2003b, 2004b) identified different outwash
remnants eastern of the Uspallata valley. As these deposits
are stratigraphically related to the Tigre Dormido rock
avalanche, this author applied relative-age criteria with the
aim to correlate these outwash deposits with those
previously identified by Espizúa (1993). Based on varnish
development, weathering degree measured on surfaces of
dacite blocks, and topographic relations; these outwash
deposits could be correlated with Uspallata and Punta de
Vacas drifts (Moreiras, 2003b).
Uspallata outwash remnants surround the deposit of
Tigre Dormido rock avalanche in the neighboring La
soltera gully. Moreover, remnants of lacustrine sediments
belonging to this rock avalanche dammed lake are also
surrounded by this outwash up stream of Soltera gully. For
this reason, Moreiras (2004b) suggested that the Tigre
Dormido rock avalanche is older than this outwash. As the
Uspallata glaciation was assigned to at least Early Middle
Pleistocene age (Espizúa and Bigazzi, 1998), the Tigre
Dormido rock avalanche must have happened before or
during this time period.
7. Placetas Amarillas events
In Placetas Amarillas, Placetas Amarilllas-1 (PA-1) and
Placetas Amarillas-2 (PA-2) landslides were identified
(Fig. 3). In the former, originated on the eastern hillslope
of the Minero peak (3813 m.a.s.l.), material moved to the
opposite side of the Placetas Amarillas gully, and then,
moved northward. Detrital material flowed down slope
through the La Quinta and Libélulas gullies 7 km from the
source area, reaching the proximity of the Uspallata
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S.M. Moreiras / Quaternary International 148 (2006) 138–148
Fig. 3. (a) Drawing showing Placetas Amarillas landslides where two deposits of PA-1 are observed (a and b), location of lacustrine sediments at sites 1
and 2 is given; and (b) Panoramic view of PA-1 and PA-2 (after Fauqué et al., 2000, 2001; Moreiras, 2004b).
railway station. The material traveled 2000 m topographically downslope, and was preserved along gullies, locally
covering Choiyoi Group outcrops.
The PA-2 event originated in the Placetas Amarillas
headwaters. In this mass-movement, chaotic material
moved approximately 1500 m horizontally and 170 m
vertically (H/L0.11).
A rock avalanche is a large bulk of mostly dry rock
debris derived from the collapse of a slope or cliff, moving
at a high velocity and for a long distance, even on a gentle
slope. These complex events can be developed in two ways:
by slide or fall of rock body continuing as a debris
avalanche; or by sudden mobilization of a debris deposit by
means of debris avalanche and debris flow (Angeli et al.,
1996). Generally, rock avalanche deposits are tongue-like,
lobate, with marked aerial delimitation, large volume, and
great extent.
PA-1 is a rock avalanche, as was initially classified by
Fauqué et al. (2000). Mechanism analysis indicate two
stages: an initial rupture and following material streaming,
with associated morphological features. Furthermore, it
seems to have been an extremely rapid movement of high
velocity traveling a large distance. PA-2 does not have some
characteristics of a primary rock avalanche. Although this
event was sourced in a steep slope (351) it does not appear
to have had a high velocity run-out of rock debris. As well,
it is not a complex event, and thus PA-2 should be classified
as a slide with a discrete displacement. Hutchinson (1988)
pointed out that there is a transition from rockslides of
moderate displacement that remains as blocks on the
surface of rupture to slides on steeper and longer surfaces
that break up into debris or transform into strurstroms.
8. Deposit characteristics
The PA-1 rock avalanche has been preserved in two
deposits: a rounded-hill with 400 m elevation eroded in the
Fig. 4. Angular and sub-angular blocks and gravels, with diameter
between 0.07 and 3 m, randomly distributed.
central part; and a relict deposit with a different elongated
morphological pattern. This deposit displays an undulating gentle surface morphology; with hollows containing
small surface ponds. Its elongate form is exposed along
the La Quinta and Las Libélulas gullies. Both deposits
may correspond to two separate episodes (Fig. 3) (Fauqué
et al., 2001). They cover an area of 7 km2 approximately
with an estimated volume of 1 109 m3.
The PA-1 chaotic deposit is poorly sorted, constituted by
blocks of different sizes embedded in a finer matrix where
the sand fraction predominates. Angular and sub-angular
blocks and gravel with diameters between 0.07 and 3 m
are randomly distributed (Fig. 4). The assemblage is of
heterogeneous composition, with volcanic, sandstone, and
rhyolitic blocks dominating that correspond to lithologies
outcropping in the source area. Rock avalanches are
characterized by uniform texture, unsorted material,
monolithologic block composition that is conditioned by
ithologies outcropping in the source area, and angular–
subangular blocks (Hewitt, 1999).
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Fig. 5. Laminated or banded aspect of the deposit. Different color layers
are associated with hydrothermal alteration.
The PA-1 deposit is matrix-supported; the matrix
proportion is more than 40%, although block-supported
structure is observed in some sectors. The deposit shows
diverse tones as a consequence of hydrothermal alteration
of the original material. Silicification and manganese
concentrations also exist. Different color-layers due to
hydrothermal alteration attribute laminated or banded
aspect (Fig. 5). Similar hydrothermal alteration was
observed in the Tigre Dormido rock avalanche (Moreiras,
2003b).
PA-2 corresponds to a lobate deposit with a high slope in
the frontal part (351). It has an extent area of 3 km2 and its
estimated volume is 2.5 108 m3. The deposit is matrixsupported with blocks of different sizes from a few
centimeters to meters in diameter. Blocks are rhyolites
with strong hydrothermal alteration: intense silication and
chlorite minerals were observed. The largest blocks are
composed by a breccia cemented by manganese, quartz and
amorphous silica.
9. Source area features
Outcrops in the source area, composed of rhyolite
porphyry and volcanite rocks, are intensively jointed.
Breccia zones related to these intense fractured rocks are
observed. Cortés et al. (1999) have also mentioned the
existence of breccias related to Piedras Blancas fault. Space
between joints is variable from millimeters to meters, and
orientations also are variable, with predominating northern–southern and northwest–southeast trends.
These outcrops are also affected by intense hydrothermal
alteration, indicated by intense silicification. Silica veins,
stockwork structures and manganese concentration are
observed. Moreover, feldspar grains in porphyries are
completely replaced by clays. Copper oxides have been also
identified such as azurite and malachite. Several breccia
boulders and clasts cemented by silica or manganese oxides
have been found.
143
PA-1 and PA-2 source areas are also characterized by
steep slopes and relief contrast. The main scarps dip 551E
and 401N, respectively.
These lithological and topographical conditions probably favored and controlled slope instability in this region.
The other rock avalanches identified, such as TD, PB-1,
PB-2 and PB-3, also originated in areas with similar
characteristics: topographical relief, lithologies affected by
hydrothermal alteration, and intense jointing probably
related to regional faults. As a consequence, structures,
lithologies, and topographic contrast seem to be key
conditioning factors for rock avalanche occurrence in this
sector of Cordon del Plata. In Voight and Pariseau (1978)0 s
opinion, several parameters such as lithology, structure,
previous slope movements and climate favor rock avalanche occurrence.
10. Evidence of paleo-lake
The extraordinary PA-1 event dammed the Minero gully
causing the formation of a barrier-lake (Fauqué et al.,
2001). A sequence of lacustrine sediments 38 m thick
has been identified at site 1 (see Fig. 3) composed by
intercalated sand and silt layers. Lower sediments are rich
in organic matter, while in the upper 4 m, at 2238 m.a.s.l.,
fine sands predominate. Relict diatomite levels deposits
were also observed in Minero gully margins (site 2) at
2120 m.a.s.l., approximately 60 m above the present gully
bed, and thus the paleo-lake was not very deep (Fig. 6).
The diatomite deposits are carbonate rich, showing violent
effervescence with HCI (1 N), and have a high content of
volcanic glass probably due to volcanic eruptions during
the lake’s existence. Samples of lacustrine sediments were
microscopically analyzed, and the diatom species determined were: Amphora sp., Achnates longipes, Central,
Cymbella af. cistula cymbiformis, Cymbella ventricosa,
Denticula, Fragilaria brevistriata var. Trigona, Gomphonema sp., Navicula sp., Neidium sp., and Nitzchia sp.
Unfortunately these species are cosmopolite, and for this
Fig. 6. Sequence of lacustrine sediments at site 1, where it can be also
observed diatomite deposits at 2200 m.a.s.l. in the Minero gully (site 2).
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S.M. Moreiras / Quaternary International 148 (2006) 138–148
reason, it was not possible to determine the paleo-climatic
environment of the lake. Furthermore, the paleo-lake
extent could not be established due to the limited outcrops
of lacustrine deposit. These deposits were probably eroded
along the Minero gully as a result of the collapse of
the dam forced by the Piedras Blancas-3 rock avalanche
(PB-3).
11. Chronology
Along La Quinta and Las Libélulas gullies, the PA-1
rock avalanche deposit is overlain by alluvial fans where
three ash levels are intercalated (Fig. 7). This volcanic
material probably comes from active volcanic centers
located southward of the study area such as Tupungatito
(5913 m.a.s.l.), San José (6111 m.a.s.l.), and Maipo
(5323 m.a.s.l.) stratovolcanoes.
Numerical dating of medium ash level by Ar40–Ar39 gave
an age of 350780 ka. Sampling of the lower ash level was
not possible due to the irregular topography. The date
suggests that, as the PA-1 rock avalanche is older than this
ash level, the maximum age for the PA-1 rock avalanche
should be Middle Pleistocene. However, the PA-2 chronology could not be inferred using stratigraphic relationships.
12. Relative dating techniques
In order to establish a chronological sequence for the
Placetas Amarillas landslides (PA-1 and PA-2), relative
dating techniques, including soil and rock varnish development, were used. These techniques are time dependent,
so they are frequently used for dating deposits (Blackwelder, 1931; Sharp and Birman, 1963; Sharp, 1969, 1972;
Birkeland, 1973; Burke and Birkeland, 1979; Colman and
Pierce, 1986; Knuepfer 1988; Wells et al., 1987a,b, Ritter,
1987).
Fig. 7. Ash level intercalated in alluvial fans, dated 350780 ka by Ar–Ar
method, overlying the PA-1 rock avalanche deposit. Filled arrow indicates
contact between alluvial fans and PA-1 rock avalanche deposit.
The PA-1 deposit shows discontinuous particles of sandy
soils with A/B/C profile. The At horizon containing
significant organic accumulation, although horizons generally erode. The horizon B reaches 20 cm thickness. It
has a moderate laminate structure, 5 YR 5/2 color, and
violently effervescence with HC1 (1 N). The horizon was
classified as a Bk, and carbonate content reaches 7.5 mg/kg.
Grain size distribution indicates that there is no clay
enrichment in comparison to horizon C. Geochemical
analysis for this horizon indicate 2.39% organic matter.
In the PA-2 surface, a similar soil profile is observed, but
the development of horizon AL is very variable, never
thicker than 5 cm. Horizon B is 40 cm thicker, 5 YR 7/2,
and has a pH of 7.65. In this horizon, 5 mg/kg carbonates
and 0.76% of organic matter were determined, both
parameters being lower than those determined in horizon
BK of the PA-1 deposit. Previous studies in the region
mentioned that carbonate content is the best indicator of
soil development (Espizúa, 1993; Moreiras, 2004b).
Rock varnish development on rhyolite blocks is greater
in the PA-1 deposit than in the PA-2 deposit (Table 1),
and differences also exist between two deposits of PA-1
(a and b). Sample areas were restricted due to intense
disintegration on the larger blocks situated on the deposit
surface, which may mask relative dating results as has been
pointed out in previous works (Moreiras, 2003b, 2004b).
13. Discussion
The PA-1 rock avalanche occurred before 350780 ka
consistent with the age obtained in this study for the tephra
layer overlying this deposit (see Fig. 7). As well, the TD
rock avalanche occurred prior to the formation of the
outwash terrace related to the Uspallata Glaciation,
assigned to the Early Middle Pleistocene according to
stratigraphic evidence (Moreiras, 2004b). Initially, Cortés
(1993) tentatively proposed that the Placetas Amarillas
Formation, corresponding to the six rock avalanche
deposits identified in the region, should be older than the
Uspallata Glaciation.
The proximity of the rock avalanche source areas
(approximately 4 km) may suggest their temporal correlation. The PA-1 and TD rock avalanches could originate
from the same triggering factor and under the same
conditions. Seismic shaking was probably the triggering
factor of these rock avalanches in accordance with seismic
history of this region (see Fig. 2). Keefer (1984) noted that
these kinds of events are commonly caused by high
magnitude earthquakes in seismic regions around the
world.
As earthquakes are generally associated with region of
active faulting in continental interior, huge landslides have
occurred in active tectonically regions (Plafker and
Ericksen, 1978; Adams, 1981; Keefer, 1984; Perucca and
Moreiras, 2003a,b). Furthermore, Hermanns and Strecker
(1999) and Hermanns et al. (2000) mentioned landslide
clusters along neotectonically active mountain fronts. Rock
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145
Table 1
Rock varnish development on blocks bigger than 10 cm randomly selected in deposit surfaces
Landslide events
N
Rock varnish development
o10
PA-1
PA-2
a
b
500
300
400
10–50
50–90
490
x
DS
x
DS
x
DS
x
DS
1.60
3.00
6.70
1.1
1.0
2.5
5.4
27.0
31.0
2.3
2.0
3.36
30.4
55.6
44.5
2.0
5.5
3.1
62.60
14.33
17.75
4.50
6.02
3.40
avalanches are absent in similar mountain fronts without
active neotectonics. The TD and PA-1 events originate in
mountain fronts affected by regional faults. These faults
are associated with the Carrera fault system which has been
active at least until Early Pleistocene (Kozlowski et al.,
1993). Moreover, evidence of Quaternary activity of these
faults has also been observed by Cortés et al. (1999).
However, the relationship between rock avalanches and
Quaternary fault activity is uncertain.
Structural controls may also favor slope instability
because the lithologies are affected by intense jointing.
Highly fractured materials are liable to rapid and intense
weathering because weakness zones enhance weathering
and water movement, and thus favor potential planes of
failure.
Paleo-climatic conditions probably favored the occurrence of these events, as the TD rock avalanche occurred
before formation of the Uspallata Glaciation outwash
terrace. Periods of exceptionally warm temperature and
severe rainfall force hillslope instability (Espizúa and
Bengochea, 1991; Gonzalez Diez et al., 1996; Poschinger
and Haas, 1997; Trauth and Strecker, 1999; Corsini et al.,
2000, 2001; Trauth et al., 2000; Hermanns et al. 2000;
Borgatti et al., 2001).
The Uspallata glaciation is the oldest Pleistocene
glaciation identified along the Rio Mendoza valley. This
glaciation was assigned to at least Early Middle Pleistocene
age because it is older than a tephra layer dated by fission
track at 360736 ka (Espizúa, 1993) and older than a tephra
layer intercalated in alluvial fans surrounding Uspallata
terminal moraine in Uspallata valley dated 170750 ka
(Espizúa and Bigazzi, 1998). Espizúa (1993) tentatively
correlated this stage with the El Soldado glaciation,
previously identified in the Aconcagua valley of Chile by
Caviedes and Paskoff (1975). Thus, Espizúa and Bigazzi
(1998) proposed that it could be correlated with the
European prepenultimate glaciation, as the previous Punta
de Vacas glaciation was correlated with penultimate
glaciation and MIS 6. Nevertheless, several Pleistocene
stadial periods have been identified in South America.
Those studied in the Strait of Magellan (Tierra del Fuego)
are estimated between 360 ka and 1.1 Myr old (Meglioli,
1992; Rabassa et al., 2000). Coronato et al. (2004a)
identified a minimum of six regional glacial advances in
Patagonia, from Late Pliocene–Early Pleistocene times to
the Late Pleistocene and Late-glacial. In North Patagonia
(411–431 South Latitude), two glaciations from Middle
Pleistocene and the Great Patagonian Glaciation of Early
Pleistocene in age have been identified. In Southern
Patagonia (441300 –521S) different drifts that correspond
to three glaciations of Middle Pleistocene age (Post-Great
Patagonian Glaciation), the Great Patagonian Glaciation,
and a Late Pliocene–Late Miocene glaciation have been
identified (Coronato et al., 2004b). Kaplan et al. (2005)
determined that different glaciations by cosmogenic
measures in Lago Buenos Aires at 461 South Latitude
correlated with MIS 2, 6, and 10 or 12.
Glacial to interglacial changes and glacial maxima are
global in Earth history, thus the glacial stages in the mid to
high latitudes of South America coincide in timing with
European and Northern American glaciations during at
least the last 200,000 yr (Ackert et al., 2003; Douglass
et al., 2005; Kaplan et al., 2004, 2005). However, local
evidence and numerical dating is basically required to
determine the precise age of the Uspallata Glaciation and
interpret the existence of a prior interglacial stage.
Paleo-climatic conditions could be determined by diatom
analysis. Nevertheless, plant species identified in these
outcroppings are cosmopolitan, and consequently climate
conditions for the dammed-paleolake could not be
determined. Future Quaternary studies in the region and
numerical dating of involved deposits could enhance
climatic influence evaluation.
In summary, relation between the occurrence of PA-1
and TD rock avalanches and regional neotectonic activity
is very probable. However, the link with Quaternary fault
activity is more doubtful because large earthquakes can be
effective at great distances (Voight and Pariseau, 1978;
Keefer, 1984). Magnitude of the historical earthquake
should have been higher than 6 according to the total
volume of displaced material (Keefer, 1984; Keefer and
Jibson, 1993).
This hypothesis, based on neotectonic relationships, will
extend seismic shakings in the region to the Middle
Pleistocene time, and may also explain the occurrence of
the younger PA-2. However, a climatic influence should not
be underestimated. These findings match Hermanns and
Strecker’s (1999) observations in other areas of the Central
ARTICLE IN PRESS
146
S.M. Moreiras / Quaternary International 148 (2006) 138–148
Andes. These authors found that the trigger mechanism for
the majority of 55 rock avalanches with volumes larger than
106 m3 and formed by the collapse of entire mountain fronts
was seismic, although the age of some major slides are
about 30 ka, which may correspond to a more humid
interval in southern South America. Whether or not rock
avalanches can be linked to Quaternary activity of the
Placetas Amarillas and Piedras Blancas faults, they were
active until Middle Pleistocene time.
14. Conclusion
This research gives an approximate chronology of the
Placetas Amarillas-1 (PA-1) and Tigre Dormido (TD) rock
avalanches enhancing knowledge about these extraordinary events. Furthermore, a younger event is identified by
geomorphology and relative age techniques. The PA-1 rock
avalanche is at least of Middle Pleistocene age, as its
deposit is overlain by an ash level dated at 350780 ka. As a
similar age was proposed for the Tigre Dormido rock
avalanche; temporal correlation of both rock avalanches is
suggested. Thus, a common seismic triggering factor
related to regional neotectonics is proposed in accordance
with regional seismic activity.
Although the occurrence of the PA-1 and TD rock
avalanches is likely related to regional neotectonic activity,
the climatic role should not be underestimated even lacking
to support the existence of a warm period prior Uspallata
glaciation. Commonly, analysis of climatic role is not easy
in seismic regions, but it must be kept in mind that slope
failures are generally influenced by a combination of
factors such as lithological, structural and climatic conditions that predispose steep mountain fronts to failure
(Schmidt and Dikau, 2004).
Future efforts are geared towards numerical dating of
rock avalanches and determining uncertainties about
climatic influence. However, stratigraphic studies carried
out in the present research are essential and will be
necessary for evaluation of accuracy on future numerical
dating.
Acknowledgements
I am grateful to Dra. Garibotti for identifying plant
species in diatom deposits and Dra. Calvedo for recognizing lichen species. I want to thank Dra. Giambiagi for
checking the original manuscript and Dr. Hermanns for his
critical review and comments that helped to improve the
original manuscript. This research was funded by a grant
from the Consejo Nacional de Investigaciones Cientı́ficas y
Tecnológicas (CONICET) and a grant from Fundación
Antorchas (Res. 48/2003).
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